81 research outputs found
On the Equivalence Theorem in the Description of the Symmetry Breaking Sector of the Standard Model
We develop an alternative formulation of the symmetry breaking sector of the
Standard Model as a gauged non-linear sigma model (NLSM) following the
philosophy of the Chiral lagrangian approach, which is the only compatible with
all the experimental and theoretical constraints. We derive the BRS symmetry of
the model and the corresponding quantum lagrangian, which is a generalization
of the standard Faddeev-Popov method, in a way which is covariant with respect
to the reparametrizations of the coset space of the NLSM. Then we use the BRS
invariance of the quantum lagrangian to state the Equivalence Theorem for the
renormalized -matrix elements calculated as a chiral expansion.Comment: LaTeX. Final version as it will appear in Nuclear Physics B (1994
CP--odd Correlation in the Decay of Neutral Higgs Boson into , , or
We investigate the possibility of detecting CP--odd angular correlations in
the various decay modes of the neutral Higgs boson including the modes of a
pair, a pair, or a heavy quark pair. It is a natural way to probe
the CP character of the Higgs boson once it is identified. Final state
interactions (i.e. the absorptive decay amplitude) is not required in such
correlations. As an illustrative example we take the fundamental source of the
CP nonconservation to be in the Yukawa couplings of the Higgs boson to the
heavy fermions. A similar correlation in the process is
also proposed. Our analysis of these correlations will be useful for
experiments in future colliders such as LEP II, SSC, LHC or NLC.Comment: 16 pages, plus 8 postscript graphs not posted befor
A miniaturized bioreactor system for the evaluation of cell interaction with designed substrates in perfusion culture
In tissue engineering, the chemical and topographical cues within three-dimensional (3D) scaffolds
are normally tested using static cell cultures but applied directly to tissue cultures in perfusion
bioreactors. As human cells are very sensitive to the changes of culture environment, it is essential to
evaluate the performance of any chemical, and topographical cues in a perfused environment before
they are applied to tissue engineering. Thus the aim of this research was to bridge the gap between
static and perfusion cultures by addressing the effect of perfusion on cell cultures within 3D
scaffolds. For this we developed a scale down bioreactor system, which allows to evaluate the
effectiveness of various chemical and topographical cues incorporated into our previously developed
tubular ε-polycaprolactone scaffold under perfused conditions. Investigation of two exemplary cell
types (fibroblasts and cortical astrocytes) using the miniaturized bioreactor indicated that: (1) quick
and firm cell adhesion in 3D scaffold was critical for cell survival in perfusion culture compared
with static culture, thus cell seeding procedures for static cultures might not be applicable. Therefore
it was necessary to re-evaluate cell attachment on different surfaces under perfused conditions before
a 3D scaffold was applied for tissue cultures, (2) continuous medium perfusion adversely influenced
cell spread and survival, which could be balanced by intermittent perfusion, (3) micro-grooves still
maintained its influences on cell alignment under perfused conditions, while medium perfusion
demonstrated additional influence on fibroblast alignment but not on astrocyte alignment on grooved
substrates. This research demonstrated that the mini-bioreactor system is crucial for the development of functional scaffolds with suitable chemical and topographical cues by bridging the gap between
static culture and perfusion culture
On the precision of the theoretical predictions for pi pi scattering
In a recent paper, Pelaez and Yndurain evaluate some of the low energy
observables of pi pi scattering and obtain flat disagreement with our earlier
results. The authors work with unsubtracted dispersion relations, so that their
results are very sensitive to the poorly known high energy behaviour of the
scattering amplitude. They claim that the asymptotic representation we used is
incorrect and propose an alternative one. We repeat their calculations on the
basis of the standard, subtracted fixed-t dispersion relations, using their
asymptotics. The outcome fully confirms our earlier findings. Moreover, we show
that the Regge parametrization proposed by these authors for the region above
1.4 GeV violates crossing symmetry: Their ansatz is not consistent with the
behaviour observed at low energies.Comment: Added more material, mostly in Sects. 7, 8 and 9, in support of the
same conclusions. Latex, 28 pages, 3 figure
Finite Temperature Pion Scattering to one-loop in Chiral Perturbation Theory
We present the pion-pion elastic scattering amplitude at finite temperature
to one-loop in Chiral Perturbation Theory. The thermal scattering amplitude
properly defined allows to generalize the perturbative unitarity relation to
the case. Our result provides a model independent prediction of an
enhanced pion-pion low-energy phase shift with the temperature and it has
physical applications within the context of Relativistic Heavy Ion Collisions.Comment: 11 pages, 2 figures. Some references and clarifying comments added
and new figures included. Final version to appear in Physics Letters
Finite temperature pion vector form factors in Chiral Perturbation Theory
We discuss the thermal behaviour of the pion vector form factors and
calculate them in one-loop Chiral Perturbation Theory. The perturbative result
is used to analyze the -dependent electromagnetic pion charge radius,
obtaining a rough estimate of the deconfinement critical temperature. Imposing
thermal unitarity, we generate the resonance pole for the form factor in
the center of mass frame. The peak height in the modulus of the form
factor decreases for increasing temperature, while its width increases and the
peak position is slightly shifted downwards for 150 MeV. These
results point in the direction suggested by many analysis of dilepton
production data in relativistic heavy ion collisions.Comment: 15 pages, 4 figures, final version to appear in Phys.Lett.B, added
references and comments, abstract change
Analyzing Power of the Proton Continuum for 150 and 200 MeV Polarized Protons on 12-C and 58,62-Ni
This work was supported by the National Science Foundation Grants NSF PHY 78-22774 A03, NSF PHY 81-14339, and by Indiana Universit
Couplings of light I=0 scalar mesons to simple operators in the complex plane
The flavour and glue structure of the light scalar mesons in QCD are probed
by studying the couplings of the I=0 mesons and to the
operators , and to two photons. The Roy dispersive
representation for the amplitude is used to determine the
pole positions as well as the residues in the complex plane. On the real axis,
is constrained to solve the Roy equation together with elastic
unitarity up to the K\Kbar threshold leading to an improved description of
the . The problem of using a two-particle threshold as a matching
point is discussed. A simple relation is established between the coupling of a
scalar meson to an operator and the value of the related pion form-factor
computed at the resonance pole. Pion scalar form-factors as well as two-photon
partial-wave amplitudes are expressed as coupled-channel Omn\`es dispersive
representations. Subtraction constants are constrained by chiral symmetry and
experimental data. Comparison of our results for the couplings with
earlier determinations of the analogous couplings of the lightest I=1 and
scalar mesons are compatible with an assignment of the ,
, , into a nonet. Concerning the gluonic operator
we find a significant coupling to both the and the
.Comment: 31 pages, 5 figure
NN,N\Delta Couplings and the Quark Model
We examine mass-corrected SU(6) symmetry predictions in the quark model
relating vector, axial-vector and strong NN and N\Delta couplings, and
demonstrate that the experimental N\Delta value is significantly higher than
predicted in each case. Nevertheless the Goldberger-Treiman relation is
satisfied in both sectors. Possible origins of the discrepancy of the quark
model predictions with experiment are discussed.Comment: 22 pg. Latex file, figures available by reques
Corrections to flat-space particle dynamics arising from space granularity
The construction of effective Hamiltonians describing corrections to flat
space particle dynamics arising from the granularity of space at very short
distances is discussed in the framework of an heuristic approach to the
semiclassical limit of loop quantum gravity. After some general motivation of
the subject, a brief non-specialist introduction to the basic tools employed in
the loop approach is presented. The heuristical semiclassical limit is
subsequently defined and the application to the case of photons and spin 1/2
fermions is described. The resulting modified Maxwell and Dirac Hamiltonians,
leading in particular to Planck scale corrections in the energy-momentum
relations, are presented. Alternative interpretations of the results and their
limitations, together with other approaches are briefly discussed along the
text. Three topics related to the above methods are reviewed: (1) The
determination of bounds to the Lorentz violating parameters in the fermionic
sector, obtained from clock comparison experiments.(2) The calculation of
radiative corrections in preferred frames associated to space granularity in
the framework of a Yukawa model for the interactions and (3) The calculation of
synchrotron radiation in the framework of the Myers-Pospelov effective theories
describing Lorentz invariance violations, as well as a generalized approach to
radiation in Planck scale modified electrodynamics. The above exploratory
results show that quantum gravity phenomenology provides observational guidance
in the construction of quantum gravity theories and opens up the possibility of
probing Planck scale physics.Comment: 49 pages, 6 figures and 4 tables. Extended version of the talk given
at the 339-th WE-Heraeus-Seminar: Special Relativity, will it survive the
next 100 years?, Potsdam, february 200
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